This invention relates in general to vehicular brake systems, and in particular is concerned with a fluid compensator provided in a hydraulic control unit to accommodate travel of a master cylinder piston.
Hydraulic braking systems for vehicles are well known- A typical hydraulic brake system includes a master cylinder, fluid conduit arranged into a desired circuit and wheel brakes. The master cylinder generates hydraulic forces in the fluid circuit by pressurizing brake fluid when the driver steps on the brake pedal. A piston in the master cylinder travels in response to the brake pedal. The pressurized fluid travels through the fluid conduit in the circuit to actuate brake cylinders at the wheel brakes and slow the vehicle.
Anti-lock braking systems (ABS) for vehicles are also well known hydraulic systems. A hydraulic control unit (HCU) or housing, containing control valves and other components such as control valves and pumps, is located between the master cylinder and the wheel brake assemblies. Through an electronic controller, the control valves and other components selectively control pressure to the wheel brake assemblies to provide a desired braking response of the vehicle.
A seal is provided about the master cylinder piston. Preferably, this seal is formed as a one-way seal commonly known as a lip seal. During a braking event, this seal can be positioned nearby a compensation port orifice of the master cylinder. Repeated passes of a seal holding pressurized fluid across the compensation port orifice as the master cylinder piston is stroked can result in deterioration of the seal and shortened effective life of the seal. Known braking systems can provide additional displacement of the master cylinder piston to move the seal away from the compensation port orifice, but such systems add extra pedal stroke to every braking event, i.e., normal (base) braking as well as anti-lock braking. Additional pedal stroke is generally not desired in many-brake systems.
This invention includes a fluid compensator provided in a hydraulic control unit of vehicular brake systems. The fluid compensator permits additional displacement of a master cylinder piston during anti-lock braking events only by storing fluid in the hydraulic control unit. The additional displacement moves a seal mounted on the master cylinder piston away from a orifice, thereby preventing repeated xe2x80x9cnibblingxe2x80x9d of the seal compensator port which can cause premature seal failure. One advantage provided by this fluid compensator is that additional pedal stroke is not present during normal base braking, since this fluid compensator has no effect during normal braking. The fluid compensator is provided in a hydraulic control unit and can be used with many variations of anti-lock brake systems.
In a first preferred embodiment, a fluid compensator to accommodate piston displacement of a master cylinder in a vehicular brake system includes a body formed from a resilient material having a gas trapped in pores. The body includes a fluid impermeable outer surface. The body can be positioned in a bore of an attenuator downstream of a pump outlet. As fluid enters the bore, the body compresses to permit additional displacement of a master cylinder piston.
In a second preferred embodiment, a fluid compensator to accommodate piston displacement of a master cylinder in a vehicular braking system includes a fluid chamber formed between an end wall of a bore mounting a slidable piston in a low pressure accumulator and a seal mounted about the piston. A fluid passageway intersects a side wall of the bore at a predetermined distance away from the end wall. At predetermined positions of the piston, fluid flow out of the fluid chamber is restricted as a suction force from a pump inlet draws the piston against the fluid passageway. In a third preferred embodiment, a fluid compensator is formed in a manner similar to the second embodiment with the inclusion of an undercut provided about an outer surface of the piston. The undercut increases the annular restriction between the piston and the bore at the area where the fluid passageway intersects the side wall.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.